Fair Value Measurements - Reconciliation of Our Investment in Restricted Equity Securities Measured at Fair Value on Quarterly Basis Using Significant Unobservable Inputs (Level 3) (Detail) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2021	Mar. 31, 2020
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Balance at beginning of period	$ 38.2
Unrealized gain (loss) included in earnings	0.7	$ (16.5)
Balance at end of period	38.9
Equity Securities \| Level 3 \| Fair Value, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Balance at beginning of period	38.2	55.9
Unrealized gain (loss) included in earnings	0.7	(16.5)
Balance at end of period	$ 38.9	$ 39.4

Fair Value Measurements - Reconciliation of Our Investment in Restricted Equity Securities Measured at Fair Value on Quarterly Basis Using Significant Unobservable Inputs (Level 3) (Detail) - USD ($)
$ in Millions

3 Months Ended

Mar. 31, 2021

Mar. 31, 2020

Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]

Balance at beginning of period

$ 38.2

Unrealized gain (loss) included in earnings

0.7

$ (16.5)

Balance at end of period

38.9

Equity Securities | Level 3 | Fair Value, Recurring